Typical solid propellants for rocket motors contain ammonium perchlorate (AP) and sometimes aluminum powder in a rubbery matrix, typically of hydroxyl-terminated polybutadiene (HTPB). The autoignition temperature of these propellants is controlled by the decomposition of ammonium perchlorate and the interactions of the decomposition products with the rubbery binder, that lead to porosity in the propellant grain. When autoignition finally occurs, around 450° F., the high surface area of the porous propellant causes rapid overpressurization in the rocket motor leading to destruction.
Conventional techniques to control autoignition of solid rocket motor propellants has included depositing a uniform, thin coating of a paraxylene polymer over the propellant grain as an ignition inhibitor as disclosed in U.S. Statutory Invention Registration No. H1, 824 to Johnsen et al (the entire content of which is expressly incorporated hereinto by reference). U.S. Pat. No. 3,052,092 to Kirkbride (the entire content of which is expressly incorporated hereinto by reference) discloses a second propellant enclosing a first propellant to act, in effect, as a barrier between the first propellant and the motor casing. The second propellant has a suitable burning rate that is lower than that of the first propellant.
Broadly, the present invention is embodied in solid propellant systems that include a main propellant and a secondary propellant in contact with the first propellant that exhibits autoignition temperatures of at least about 100° F. lower than the autoignition temperature of the main propellant. The secondary propellant of the present invention is most advantageously employed with conventional AP-containing solid propellant formulations as the main propellant, especially formulations containing both AP (an energetic solid) mixed with a compatible binder. In especially preferred forms of the present invention, the secondary propellant will include a nitramine which is at least one selected from nitroguanidine (NQ), cyclotrimethylene trinitramine (RDX) and cyclotetramethylenetetranitramine (HMX), and a binder which is at least one selected from HTPB, hydroxyl-terminated polyethylene (HTPE) or glycidyl azide polymer (GAP). Most preferably, the secondary propellant will include a combination of nitramines which includes NQ and one of RDX or HMX.
The secondary propellant is most preferably in physical contact with a localized region of the main propellant near the motor casing. In this regard, most preferably the secondary propellant is in the form of a relatively narrow (i.e., as measured in the elongate axis of the motor casing) strip or ring circumferentially positioned near the motor casing wall in contact with the main propellant. The localized positioning of the strip of secondary propellant provides a critical mass such that heat generation exceeds heat losses. As such, reaction violence is dramatically reduced since the main propellant is not significantly decomposed when the secondary propellant autoignites.
These and other aspects and advantages will become more apparent after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.